In the process of xerography, a light image corresponding to the original to be copied is typically recorded in the form of a latent electrostatic image upon a photoconductive member. This latent image is developed, that is to say, made visible, by the application of a pigmented thermoplastic resin, commonly referred to as toner. The thus developed latent image is thereafter transferred from the photoconductive member onto a copy medium, such as for example, a copy sheet or portion of a roll of paper. The copy medium is subsequently passed through a fusing apparatus which affixes the image onto the medium and is later discharged from the machine as a final copy.
One approach to fixing the toner particles onto the copy sheet has been to pass the copy sheet with toner images thereon through a fusing nip formed by a heated fuser roll and a backup roll. As it passes therethrough, the copy sheet is simultaneously pressed and heated so that the toner becomes softened and firmly attached to the copy sheet.
In such a fusing arrangement, opening and closing of the fuser nip is commonly controlled by a cam rotatable in synchronization with movement of the copy sheets. Frequently therefore, during the processing of variable length sheets (i.e., sheets of different length than the selected standard which is the longest anticipated sheet length), the rollers are allowed to remain in contact during periods in which no copy sheet is disposed therebetween. This prolonged direct contact commonly results in the overheating of the backup roll. Such overheating of the roller may result in a paper jam as the copy sheet will tend to follow the backup roll rather than continuing along the intended paper path beyond the fuser station. This backup roll sticking problem is especially aggravated during the fusing of duplex copies (i.e., sheets with toner copy on both sides) due to the cohesive nature of the toner. Overheating of the roller surface may also result in a phenomena referred to in the printing art as "offset" wherein toner adheres to the roller surface and is transferred to the next copy sheet.
Substantial damage may also be caused to the rollers from entrapment of copy sheets between the heated fuser roll and the backup roll when there is a loss of power during a copying operation. It is desirable therefore that the fuser rollers remain in an open position during standby mode and similarly during a copying operation that the fusing nip be closed only when there is paper between the rollers. Thus, it is desirable that the fusing nip be opened during the intersheet gap that may exist between adjacent copy sheets and be closed only when the next sheet arrives. Likewise, when a paper jam is detected, control of the closure mechanism is desirable so that fuser roller opening may be effected and damage to the rollers avoided. Further, it is desirable to open the fuser nip to facilitate maintenance. IMB TDB "Fuser Configuration" (5/81, pp. 5622-5623) describes the details of a pivoting and latching mechanism useful for this purpose.
Prior devices frequently control the operation of a cam and thereby the opening and closing of the fuser rollers. For example, U.S. Pat. No. 4,038,026 utilizes a spring clutch to actuate a roll closure cam in response to copy sheet position sensing. Such devices, while providing for the interruption of fuser roller closure upon the occurrence of certain conditions, are dependent on the coasting effect of the apparatus. It is necessary therefore, that the clutch act as a single-turn clutch, so that when interrupted, the cam does not stop immediately but rather continues to travel through an angle sufficient to effect the opening of the rollers. Such an arrangement is especially disadvantageous in the case of a loss of power during a copying operation as the machine often will not coast far enough to operate the cam to effect opening of the rollers. IBM TDB "Electromagnetically Latchable Damper Assembly" (7/80, pp. 474-475) describes an interruptable linkage arrangement which allows for the fuser rollers to be opened via deenergizing an electromagnet. This arrangement allows for fuser roller opening upon a power loss, but deenergization of the solenoid is not otherwise associated with the occurrence of other conditions upon which opening the fuser rollers is desired.
Accordingly, it is a principal object of this invention to provide an improved xerographic toner fixing apparatus.
It is another object of this invention to provide a toner fixing apparatus which allows the operation of heated fuser roll and backup roll opening and closing under mechanical cam control for standard operations, but which permits overriding or interrupting the cam closure mechanism upon the occurrence of certain conditions.
Another object of this invention is to provide a toner fixing apparatus which provides for the operation of heated fuser roll and backup roll opening and closing independent of machine coast.
A further object of this invention is to provide a toner fixing apparatus capable of fusing toner images onto a copy sheet without toner offset.
Still another object of this invention is to provide a toner fixing apparatus which avoids fuser roller damage caused by copy sheet entrapment.